Capacitive measuring system

ABSTRACT

The invention relates to a measuring device including at least one measuring probe, sequentially applying a controlled supply voltage between the measuring probe and a reference element, and integrating accumulated electric charges on the measuring probe. The device also includes at least one auxiliary measuring probe, which is also sequentially linked to a controlled electric supply and to charge integrating means. The auxiliary measuring probe has a capacity, in relation to a potential detection zone, which is different from the main measuring probe. Comparative use of signals respectively emitted by the two measuring probes enables the influence of the main measuring probe to be determined.

BACKGROUND OF THE INVENTION

The present invention relates to the field of sensors.

FIELD OF THE INVENTION

More precisely, the present invention relates to a measuring device thatmakes use of an indirect measurement of the permittivity between twoelectrically conducting bodies forming a measurement probe and areference element, for example a reference probe, respectively.

DESCRIPTION OF THE RELATED ACT

Document WO-0025098 discloses a device whose basic structure is shownschematically in the appended FIG. 1.

This device comprises two electrically conducting bodies, constituting ameasurement probe 10 and a reference probe 20 respectively, power supplymeans 30 capable of delivering a DC voltage of controlled amplitude, anintegrating stage 50 that includes a capacitor switching system 53, andcontrol means 40 suitable for cyclically defining, at a controlledfrequency, a series of two sequences:

-   -   a first sequence T1 during which the power supply means 30 are        connected to the measurement probe 10 in order to apply an        electric field between the measurement probe 10 and the        reference probe 20 and to accumulate electric charges on the        measurement probe 10; and then    -   a second sequence T2, during which the power supply means 30 are        disconnected from the measurement probe 10 and the latter is        connected to a summing point of the integrating stage 50 in        order to transfer charges into the integrating stage 50 and        obtain, as output by the latter, a signal representative of the        permittivity that exists between the measurement probe 10 and        the reference probe 20.

More precisely still, according to document WO-0025098, the integratingstage 50 comprises an operational amplifier 51, a first integratingcapacitor 52 mounted in a feedback loop onto this amplifier 51 and asecond capacitor 53 switched between the output and the input of theoperational amplifier 51 at the rate of the sequences controlled by thecontrol means 40, in such a way that, in the steady equilibrium state,the operational amplifier 51 delivers, as output, an equilibrium voltageof E.Cs/C53, in which relationship −E denotes the amplitude of thevoltage at the terminals of the power supply means 30 and Cs and C53denote the values of the capacitances defined, on the one hand, betweenthe measurement probe 10 and the reference probe 20 and, on the otherhand, the switched second capacitor 53, respectively.

The power supply means 30 and the second capacitor 53 are switched bychange-over switches 42, 43 controlled by a time base 41.

The operation of this known device is essentially the following.

Let us suppose that initially the integrating capacitor C52, theswitching capacitor C53 and the capacitor Cs that is formed between themeasurement probe 10 and the reference probe 20 are each completelydischarged, i.e.:

-   -   QC52=0;    -   QC53=0; and    -   QCs=0.

During the first sequence T1, the capacitor Cs is charged to the supplyvoltage delivered by the module 30, which is assumed here to be equal to−E.

Therefore, at the end of the sequence T1:

-   -   QCs=−E.Cs;    -   QC52=0;    -   QC53=0.

During the next sequence T2, the charges are transferred from Cs to C52;i.e. the charges being conserved and Cs and C53 both being connected tothe inverting input of the operational amplifier 51 of zero virtualimpedance:

-   -   −E.Cs=Vs2.C52,        Vs2 being the output voltage of the operational amplifier 51        during the sequence T2.

During the next sequence T1, the two capacitors C52 and C53 are placedin series. Thus:Vs=Vs2.C52/(C52+53)=QC53/C53=QC52/C52,

-   i.e. QC53=[Vs2.C52/(C52+C53)].C53    -   =[Vs2/(1+C53/C52).C53-   i.e., if C52=nC53>>C53,    -   QC53≈Vs2.C53.

At the next sequence T2, the charges contained in C53 are in oppositionwith those Cs. The remaining part of the charges of Cs is transferredinto C52, etc.

The output voltage Vs, output by the operational amplifier 51,progressively increases until reaching a voltage:VS _(equilibrium) =QC53/C53,such that QC53=Vs_(equilibrium).C53=−E.Cs.

Thus, after x iterations, the device reaches an equilibrium state at thesumming point. The charges QC53 and C53 compensate for the charges onthe probe Cs.

As soon as a change in capacitance Cs is detected, the increase (ordecrease) of charges on Cs will charge (or discharge) the capacitor C52.

Thus, in the steady state, the switching capacitor C53 will balance thevariations in charges on the probe Cs.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is now to propose a novel devicethat again adopts the concept described in document WO-0025098, butwhose performance is superior to that of the known prior devices.

More precisely still, the object of the present invention is to proposenovel means for better identifying the environment of the measurementprobe, in order to improve the detection of a transient phenomenon, forexample to eliminate the effect of a permanent obstacle inserted betweenthe measurement probe and the region in which a transient phenomenonoccurs.

In this context, the present invention is especially, but notexclusively, applicable in the detection of a person or of an object ona motor vehicle seat.

This object is achieved within the context of the present invention by adevice comprising at least one main measurement probe, means capable ofsequentially applying a controlled supply voltage between the mainmeasurement probe and a reference element and means capable ofintegrating the electrical charges accumulated on the main measurementprobe, characterized in that it furthermore includes at least oneauxiliary measurement probe connected, also sequentially, to controlledpower supply means and to charge integration means, said auxiliarymeasurement probe having, with respect to a potential detection region,a different capacitance from the main measurement probe, in such a waythat it is possible, by comparing the signals emanating from the twomeasurement probes respectively, to determine the influence of the mainmeasurement probe.

According to a first embodiment of the present invention, the auxiliarymeasurement probe has a controlled area that is small compared to themain measurement probe.

According to a second embodiment of the present invention, the auxiliarymeasurement probe is located at a different distance from the potentialdetection region than the main measurement probe.

According to a third embodiment of the present invention, the auxiliarymeasurement probe lies in the same plane, at a different distance fromthe reference element, as the main measurement probe.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, objects and advantages of the present invention willbecome apparent on reading the detailed description that follows, inconjunction with the appended drawings given by way of non-limitingexamples in which:

FIG. 1, described previously, shows schematically a device according tothe prior art disclosed in document WO-0025098;

FIG. 1 a shows schematically. a device according to the first embodimentof the present invention.

FIG. 2 shows schematically the detection of a passenger on a vehicleseat, using measurement probes according to the document WO-0025098;

FIG. 3 shows schematically the same device in the case of an obstacleinserted between the measurement probes and the detected body;

FIG. 4 shows schematically, in a plan view, a main measurement probe andan auxiliary measurement probe in accordance with a first embodiment ofthe present invention;

FIG. 5 shows the same main and auxiliary measurement probes according toa first embodiment of the present invention, in the case of thedetection of a body having a different area from that of FIG. 4;

FIG. 6 shows schematically, in a sectional view, a main measurementprobe and an auxiliary measurement probe in accordance with a secondembodiment of the present invention;

FIGS. 7 and 8 show schematically, in a sectional view and a plan viewrespectively, a main measurement probe and an auxiliary measurementprobe in accordance with a third embodiment of the present invention;

FIGS. 9 and 10 show schematically an example of a power supply for themeasurement probes illustrated in FIGS. 7 and 8 and of the resultingdetection;

FIG. 11 shows a plan view of the probes according to another embodimentof the present invention;

FIG. 12 shows a plan view with the scale enlarged in the transversedirection and compressed in the longitudinal direction of the sameprobes;

FIG. 13 shows a cross-sectional view of the same probes and illustratesmore precisely the field lines according to one particular embodiment;and

FIG. 14 illustrates a graph of the results obtained using the probesillustrated in FIGS. 11 to 13, which graph makes it possible todiscriminate between various detection configurations.

DETAILED DESCRIPTION OF THE INVENTION

For the sake of simplification, the detailed description that followswill be given with reference to the detection of a person on a motorvehicle seat.

However, the present invention is not limited to this particularapplication.

FIG. 2 shows schematically two measurement probes 10 incorporated intothe seat cushion 90 of a motor vehicle seat for the possible detectionof a person P.

As described in document WO-0025098, the person P can be detected bysequentially applying a controlled electrical voltage between themeasurement probes 10 and a reference element, such as the chassis ofthe motor vehicle, and then by integrating the electric charges thusaccumulated on the measurement probes 10.

As a variant, the two probes labeled 10 in FIG. 2 may serve in the caseof one of them as measurement probe and in the case of the other asreference element.

FIG. 3 shows schematically the same measurement probes 10 for thedetection of the same person P, but in the case in which an obstacle 0,such as a bead seat cover or a towel, is inserted between the seatcushion, and therefore the probes 10, and the person P.

A person skilled in the art will understand that such an obstacleincreases the distance between the probes 10 and the person P (thisdistance goes from e1 to e2) and consequently reduces the output signalfrom the detection device defined in document WO-0025098.

Without taking any particular precaution, there is therefore a risk ofsuch an obstacle O falsifying the detection, likening the person P to asmaller mass than that of FIG. 2.

As indicated above, the object of the present invention is to proposemeans for eliminating this difficulty.

According to a first embodiment shown schematically in FIGS. 1 a, 4 and5, the present invention provides an auxiliary measurement probe 100having a markedly smaller area than that of the main measurement probein FIG 1 a, the remaining elements shown are described above withreference to FIG. 1.

According to the representation given in FIGS. 4 and 5, the auxiliarymeasurement probe has a square outline. However, the invention is notlimited to this particular arrangement. The auxiliary measurement probe100 may have any other suitable geometry such as, for example, acircular outline.

The auxiliary measurement probe 100 is located at the same distance fromthe potential detection region, for example the upper surface of a seat,as the main measurement probe 10.

The area and the location of the auxiliary measurement probe 100 aresuch that the latter always experiences the same external influenceduring the occurrence of a transient external phenomenon, irrespectiveof the magnitude of this phenomenon.

For example in the case of the detection of a person P, the area and thelocation of the auxiliary measurement probe 100 are such that the latteralways lies entirely beneath the person P when he is sitting on theseat.

On the other hand, the area and the location of the main measurementprobe 10 are such that the area of this main measurement probe 10,influenced by the transient external phenomenon, depends on themagnitude of this phenomenon, for example it depends on the corpulenceof the person P in the case of the detection of a person on a motorvehicle seat.

As a non-limiting example, the auxiliary measurement probe 100 may becentered on the detection region and have a larger transverse dimension,of around a few centimeters, for example less than 3 cm and preferablyless than 1 cm, and a total area of less than a few square centimeters,for example less than 9 cm² and preferably less than 4 cm².

In contrast, the main measurement probe 10 preferably has at least onedimension greater than the largest possible dimension of the body P thatcan be detected.

The main measurement probe 10 may have any geometry, such as for examplea sinusoidal or rectangular geometry. In the latter case, an area ofaround a few cm by several dm, for example around a few cm, such asaround 5 cm, by more than 30 cm, preferably more than 40 cm, isdeveloped.

The use of the auxiliary electrode illustrated in FIGS. 4 and 5 allowsthe measurement to be normalized with respect to the distance from thebody P.

This is because the signal resulting from the integration of the chargesaccumulated on the auxiliary measurement probe 100 can be used todetermine the distance separating the body P from the auxiliarymeasurement probe 100, since the size of the body P has no influence onthis measurement.

On the other hand, the signal resulting from the integration of thecharges accumulated on the main measurement probe 10, normalized by theresult coming from the auxiliary measurement probe 100, can be used fordirectly obtaining reliable information representative of the size ofthe body P.

According to a second embodiment shown schematically in FIG. 6, thepresent invention provides an auxiliary measurement probe 100 located ata different distance from the potential detection region, for examplethe upper surface of a seat cushion, than the main measurement probe 10,but preferably having an area identical to that of the main measurementprobe. The auxiliary measurement probe 100 is also close to the mainmeasurement probe 10.

The difference in distance between the two probes 100 and 10, relativeto the body to be detected, must be constant.

Thus, the difference in influence of the body P on the two respectiveprobes 10 and 100 depends only on the difference in distance between thebody P and these probes 10 and 100.

Let:

-   -   S1 be the area of the main measurement probe 10;    -   S2 be the area of the auxiliary measurement probe 100;    -   e be the distance separating the measurement probe 10 from the        body P; and    -   a be the additional distance separating the main measurement        probe 10 and the auxiliary measurement probe 100 from the body        P;

then the following are obtained:

-   -   a capacitance C1=K(S1/e) on the main measurement probe 10; and    -   a capacitance C2=K[S2/(e+a)] on the auxiliary measurement probe        100.

The combination of the above two expressions therefore makes it possibleto determine the distance e and subsequently to factor out this distancein the measurement.

Of course, a similar detection may be performed with measurement probes10 and 100 having any geometry and areas S1 and S2 that are different,but in a known ratio, which are positioned at a relative distance a thatis also known.

As indicated above in the case of FIGS. 4 and 5, the main measurementprobe 10 preferably has at least one dimension greater than the largestpossible dimension of the body P that can be detected. The mainmeasurement probe 10 may also be in accordance with the provisionsdescribed above with regard to FIGS. 4 and 5.

According to a third embodiment shown schematically in FIGS. 7 to 10,the present invention provides a main measurement probe 10 and anauxiliary measurement probe 100 that is placed at a different distance(unsymmetrical probes) from a reference element 110.

Thus, according to the active measurement probe, 10 or 100, thedistribution of the electrical field varies (see especially FIGS. 9 and10) and therefore the influence of the body P to be detected on thissame probe varies according to the distance.

The distance e that separates the body P to be detected from themeasurement probes 10 and 100 can therefore be determined by thecombined use of the signals emanating from these two measurement probes10 and 100, for example by a ratio of the two capacitances C1 and C2measured on these two probes.

By way of non-limiting example, as illustrated in FIGS. 7 and 8, thetwo, main 10 and auxiliary 100, measurement probes may be coplanar withthe reference element 110. According to the non-limiting representationgiven in FIGS. 7 to 10, the auxiliary probe 100 is located between themeasurement probe 10 and the reference element 110. Typically, thecenter-to-center distance l1 between the two measurement probes 10 and100 is around a few millimeters and the center-to-center distance l2between the auxiliary probe 100 and the reference element 110 is arounda few centimeters, but at least twice l1.

The main measurement probe 10 and auxiliary measurement probe 100preferably have identical areas, but they may have any geometry, forexample a rectangular geometry. The reference element 110 may also havean area identical to the main 10 and auxiliary 100 measurement probes.However, as a variant, the main 10 and auxiliary 100 measurement probesmay have areas that differ in a known ratio.

Here again, the main measurement probe 10 at least preferably has atleast one dimension greater than the largest possible dimension of thebody P that can be detected.

As shown schematically in FIGS. 9 and 10, when one of the two probes, 10or 100, is active, the other probe, 100 or 10, may itself serve asauxiliary reference element.

It should be noted that, for the purpose of the present invention, themain 10 and auxiliary 100 measurement probes are each sequentiallyconnected to a power supply of known amplitude and then the electriccharges accumulated on these probes are integrated, preferably usingmeans similar to those defined in document WO-0025098 (and describedwith regard to FIG. 1). The power supply means and the chargeintegration means may be the same for different probes, 10 and 100. Inthis case, switching/multiplexing means alternately switch the probes 10and 100 to the terminals of these means. As a variant, it is possible toprovide different power supply means and different charge integrationmeans for the various probes 10 and 100.

For the purpose of the present invention, the reference element may beformed from a reference probe or else from a ground formed for exampleby the neighboring metal ground or earth, for example the chassis of amotor vehicle.

A person skilled in the art will understand that the various embodimentsaccording to the present invention, described above, allow two mutuallyindependent measurements to be made, under the same conditions, of thesame transient phenomenon and therefore make it possible to decouple thetwo phenomena from influences formed by the area and the distance of thebody P to be detected.

Of course, the present invention is not limited to particularembodiments that have just been described, but rather it extends to anyvariant in accordance with its spirit.

The present invention may have a large number of applications. Mentionedpreviously was the detection of someone using a motor vehicle seat,especially for actuating a safety airbag system. However, the presentinvention is not limited to that particular application. For example,the present invention may also relate, inter alia, to the fields ofanti-intrusion detection or else fluid level detectors.

The alternative embodiment according to the present inventionillustrated in the appended FIGS. 11 to 13 will now be described.

These figures again show the three probes, labeled 10, 100 and 110respectively.

The functions of these three probes 10, 100 and 110 may vary dependingon the configuration in which they are used.

In principle, the probe 110 serves as reference probe. In this context,the probe 10 constitutes the main measurement probe while the probe 100constitutes the auxiliary measurement probe.

However, during another operating phase according to the presentinvention, the probe 10 may constitute the main measurement probe, whilethe probe 100 serves as reference probe.

According to the embodiment shown in FIGS. 11 to 13, each probe 10, 100and 110 is elongate. Its length L is typically greater than 10 times itswidth, very preferably its length L is typically greater than 20 timesits width.

According to a notable first feature of the embodiment shown in FIGS. 11to 13, the probe 100 has a U-shaped configuration. Thus, the probe 100has two mutually parallel main strands 102, 104, placed respectively oneither side of the probe 10. In other words, the probe 100 surrounds theprobe 10. For this purpose, the two strands 102, 104, at one of theirends, are joined together by a linking element 103.

The Applicant has determined that, owing to the aforementioned features,the electrode 10 is very insensitive to the edge effects of the electricfield and gives signal information only when the passenger is very closeto the probe. In contrast, the electrode 100 is very sensitive to theedge effects of the electric field and gives signal information evenwhen the passenger is very far from the probe (for example, typically upto around twenty centimeters from the probe).

According to an important second feature of the embodiment shown inFIGS. 11 to 13, the probe 10 and the probe 100 have widths that varyalong their length.

More precisely still, each of the probes 10 and 100 preferably has threeportions, namely a central portion 16, 106 and two end portions 18, 19;108, 109.

Preferably, the end portions 18, 19 on the one hand, and 108, 109 on theother, have identical widths for a given probe 100 or 200.

More precisely still, the probe 100 preferably has a central portion 16of length L16 and of large width l16 and two end portions 18, 19 oflength L18 and L19 and of small width l18, l19 smaller than l16.

Typically, but not limitingly, l16=4l18=4l19.

Also typically, L16=2L18=2L19.

Preferably, the probe 100 has a central portion 106 of length L106 andof small width l106 and two end portions 108, 109 of length L108, L109and of large width l108, l109 greater than l106.

Typically, l108=l109=2l106=2l18=2l19.

Typically, L106=2L108=2L109.

Owing to the geometric characteristics that have just been mentioned,when a voltage is applied alternately or simultaneously between theprobes 10 and 110 on the one hand, and 100 and 110 on the other, theprobe 10 is very sensitive to a centered external element, that is tosay one placed opposite the central portion 16, whereas the probe 100 isvery sensitive to an off-centered element, that is to say one placedopposite the end portions 108, 109.

According to a third significant feature of the invention, illustratedin FIGS. 11 to 13, each probe 10, 100 and 110 is nonrectilinear. In thiscase, each probe 10, 100, 110 is formed from various segments, that areindividually rectilinear, but joined in pairs via their ends bytransition elements formed from dihedra whose concavities alternate,that is to say the concavities are directed alternately in one directionand then in the other. Thus, the probes 10, 100 and 110 are in the formof zig-zagged corrugations.

Such a geometry permits extension by deformation of the support.

This feature is particularly important when the probes 10, 100 and 110are incorporated in a vehicle seat. This is because this geometrypermits extention of the probes when a driver or passenger sits on theseat.

It should also be noted that the distance separating the probe 110 fromthe probe 10 or from the probe 100 is preferably greater than thedistance that separates the probes 10 and 100 from each other.

Furthermore, the supply voltage application means are suitable forsequentially applying a voltage between the terminals 10 and 100 in oneoperating phase and between the probe 110 and each of the two probes 10and 100 during another operating phase.

In FIG. 13, the field lines obtained when a voltage is applied betweenthe probes 10 and 100 are labeled C1, whereas the field lines obtainedwhen a voltage is applied between the probe 110 and each of the twoprobes 10 and 100 are labeled C2.

On examining FIG. 13 it may be seen, when a voltage is applied betweenthe probes 10 and 100, the range of detection is greatly reduced becausethe field lines C1 are highly curved.

In contrast, when the voltage is applied between the probe 110 and eachof the probes 10 and 100, the detection range is much larger since thefield lines C2 are approximately orthogonal to the supports of theprobes.

By way of nonlimiting example, the probe 110 may be placed approximately2 cm from the probes 10 and 100.

As indicated above, for the purpose of the present invention, thesignals emanating from the respective various probes are used in acomparative manner.

Within the context of the embodiment shown in FIGS. 11 to 13, thesignals obtained on the probes 10 and 100, and the signals obtained bysumming on the two probes 10 and 100, may thus be compared with thesignal from the probe 10 or with the signal from the probe 100, etc.

A person skilled in the art will understand that the present inventionthus provides a large number of comparison choices.

Even more precisely, but non-limitingly, the present invention may, forexample, make use of one of the following ratios:

-   -   U/C, U representing the signal taken off the probe 100 when a        supply voltage is applied between, on the one hand, the probes        10 and 110 joined together and serving as reference probe and,        on the other hand, the probe 100, while C represents the signal        taken off the probe 10 when a supply voltage is applied between,        on the one hand, the probes 100 and 110 joined together and        serving as reference probe and, on the other hand, the probe 10;    -   UC/C, UC representing the signal taken off the probe 100 when a        supply voltage is applied between the probe 110 and        simultaneously the probes 10 and 100;    -   UC/U;    -   CU/U, CU representing the signal taken off the probe 10 when a        supply voltage is applied between the probe 110 and        simultaneously the probes 10 and 100;    -   CU/U.

By way of non-limiting example, by comparing the sum of the signalsUC+CU defined above with the signal C defined above, which correspondsto the graph illustrated in FIG. 14, it is possible to ditinguish fourregions:

-   -   region A, which corresponds to the absence of an external        influence element, for example an empty seat;    -   region B, which corresponds to an environment without a wet        obstacle, for example a seat occupied without a wet obstacle;    -   region C, which corresponds to an environment occupied with a        wet obstacle, for example a seat occupied with a wet obstacle;        and    -   region D of an environment remotely occupied, for example a seat        occupied with a passenger away from the seat.

Of course, the present invention is not limited to this particularmethod of operation but extends to any variant in accordance with itsspirit.

1. A measuring device comprising: at least one measurement probe (10),means (30) for sequentially applying a controlled supply voltage betweenthe measurement probe (10) and a reference element (20); means (50) forintegrating the electrical charges accumulated on the measurement probe(10), at least one auxiliary measurement probe (100) connected, alsosequentially, to controlled power supply means (30) and to chargeintegration means (50), said auxiliary measurement probe (100) having,with respect to a potential detection region, a different capacitancefrom the main measurement probe (10), in such a way that it is possible,by comparing the signals emanating from the two measurement probes (10,100) respectively, to determine the influence of the main measurementprobe.
 2. The device as claimed in claim 1, wherein the auxiliarymeasurement probe (100) has a controlled area that is small compared tothe main measurement probe (10).
 3. The device as claimed in claim 2,wherein the auxiliary measurement probe (100) lies at the same distancefrom the potential detection region, for example the upper surface of aseat, as the main measurement probe (10).
 4. The device as claimed inclaim 2 or claim 3, wherein the area and the location of the auxiliarymeasurement probe (100) are such that the latter always experiences thesame external influence when a transient external phenomenon occurs,irrespective of the magnitude of this phenomenon.
 5. The device asclaimed in claim 2, wherein the area and the location of the mainmeasurement probe (10) are such that the area of this main measurementprobe (10) influenced by the transient external phenomenon depends onthe magnitude of this phenomenon.
 6. The device as claimed in claim 2,wherein the auxiliary measurement probe (100) is centered on thedetection region.
 7. The device as claimed in claim 2, wherein theauxiliary measurement probe (100) has a larger transverse dimension, ofaround a few centimeters.
 8. The device as claimed in claim 7, whereinthe transverse dimension is less than 1 cm.
 9. The device as claimed inclaim 2, wherein the auxiliary measurement probe (100) has a total areaof less than a few square centimeters.
 10. The device as claimed inclaim 9, wherein the total area is less than 4 cm².
 11. The device asclaimed in claim 2, wherein the measuring device includes means forusing the signal emanating from the integration of the chargesaccumulated on the auxiliary measurement probe (100) to determine thedistance separating a body (P) from the auxiliary measurement probe(100) and then consequently to normalize the measurement obtained fromthe main probe (10).
 12. The device as claimed in claim 1, wherein theauxiliary measurement probe (100) is located at a different distancefrom the potential detection region than the main measurement probe(10).
 13. The device as claimed in claim 12, wherein the auxiliarymeasurement probe (100) has an area identical to that of the mainmeasurement probe (10).
 14. The device as claimed in claim 12, whereinthe auxiliary measurement probe (100) has a different area from that ofthe main measurement probe (10), but in a known ratio relative to thelatter.
 15. The device as claimed in claim 12, 13 or 14, wherein theauxiliary measurement probe (100) is close to the main measurement probe(10), so that the difference in influence of an external body (P) on therespective two probes (10, 100) depends only on the difference indistance between the body (P) and these probes (10, 100).
 16. The deviceas claimed in claim 12, wherein the measuring device includes means forcombining the signals detected on the two measurement probes (10, 100)in order to determine the distance (e) between the main probe (10) and abody (P) to be detected and subsequently to factor out said distance inthe measurement.
 17. The device as claimed in claim 1, wherein theauxiliary measurement probe (100) and the main measurement probe (10)are asymmetric with respect to a reference element (110).
 18. The deviceas claimed in claim 17, wherein the auxiliary measurement probe (100) islocated at a different distance from a reference element (110) than themain measurement probe (10).
 19. The device as claimed in claim 17 orclaim 18, wherein it includes means for determining the distance (e)separating a body (F) to be detected from the measurement probes (10,100) by combined use of the signals emanating from these two measurementprobes (10, 100), for example by a ratio of the two capacitancesmeasured on these two probes.
 20. The device as claimed in claim 17,wherein the two, main (10) and auxiliary (100), measurement probes arecoplanar with the reference element (110).
 21. The device as claimed inclaim 17, wherein the main (10) and auxiliary (100) measurement probeshave identical, for example rectangular, areas.
 22. The device asclaimed in claim 17, wherein the reference element (110) has an areaidentical to the main (10) and auxiliary (100) measurement probes. 23.The device as claimed in claim 17, wherein the main (10) and auxiliary(100) measurement probes have different areas in a known ratio.
 24. Thedevice as claimed in claim 17, wherein when one of the two measurementprobes (10, 100) is active, the other measurement probe (100, 10) itselfserves as auxiliary reference element.
 25. The device as claimed inclaim 1, wherein the main measurement probe (10) has at least onedimension greater than the largest possible dimension of the body (P)that can be detected.
 26. The device as claimed in claim 1, furthercomprising: a probe (100) of U shaped configuration comprising twomutually parallel main strands (100, 204) placed respectively on eitherside of another probe (10).
 27. The device as claimed in claim 1,further comprising: at least one probe (10, 100) having a width thatvaries along its length.
 28. The device as claimed in claim 27, furthercomprising: at least one probe (10, 100) having three portions, namely acentral portion (16, 106) and two end portions (18, 19; 108, 109). 29.The device as claimed in claim 27, further comprising: at least oneprobe (10) having a central portion (16) of large width (116) and twoend portions (18, 19) of small width (118, 119).
 30. The device asclaimed in claim 27, further comprising: at least one probe (100) havinga central portion (106) of small width (1106) and two end portions (108,109) of large width (1108, 1109).
 31. The device as claimed in claim 1,further comprising: at least one nonrectilinear probe (10, 100, 110).32. The device as claimed in claim 31, further comprising: at least oneprobe (10, 100, 110) formed from various rectilinear segments joinedtogether in pairs via their ends by transition elements of the concavedihedron type with alternating concavities.
 33. The device as claimed inclaim 1, further comprising: a first set of two probes (10, 100) thatare located at a defined distance from each other and a third probe(110) located at a greater distance from the two first mentioned probes(10, 100) than the gap that exists between said two probes.
 34. Thedevice as claimed in claim 23, further comprising: means forsequentially applying a voltage between, on the one hand, the two firstmentioned probes (10, 100) and, on the other hand, between the thirdprobe (110) and each of the two first mentioned probes (10, 100),respectively.